Many gas turbine compressor rotors contain a part known as an air duct. The air duct includes a hollow tube that is located inside the rotor and is concentric with the axis of rotor rotation. The function of the air duct is to separate air that flows along the engine centerline from the air that is in contact with the structural components of the rotor. Such separation facilitates avoiding thermal gradients in the rotor parts and matching the thermal growth of the rotor to the stationary components. The aft end of the air duct is typically fastened to the compressor rotor spool shaft with a diametrical interface fit or a screw thread. The forward end of the air duct is in a slight slip fit with a small radial clearance with the forward rotor components, usually the stage one or stage two disk.
With an interference fit at the air duct aft end, the air duct must be cooled before it can be assembled into the spool shaft. When the part temperatures normalize, there may be difficulty aligning the front of the air duct with the stage two disk. This alignment is important to keep the rotor straight thus maintaining good dynamic balance at rotor operating speed. It may also be difficult to disassemble the air duct from the rest of the rotor because of the interference fit.
If the air duct is threaded into the spool shaft, internal threads are required in the mating spool shaft. These threads may become initiation sites for cracks that can result in low part life due to the stress concentration that is inherent in screw threads. These threads may be damaged when disassembling a rotor during maintenance procedures, resulting in an unserviceable spool shaft or compressor rear shaft. Also, a threaded air duct may have problems aligning the front end with the forward rotor components.
It would be desirable to provide an air duct for a gas turbine compressor rotor in which the aft end easily couples to the compressor spool shaft and the front end of the air duct is aligned with the stage one or stage two disk. It would also be desirable to provide an air duct that for maintenance is also easy to disassemble from the rotor without damaging the spool shaft or compressor rear shaft.